1. Field of the Invention
The present invention relates to a transition part between two optical waveguides. More precisely, the invention relates to a transition part between two such waveguides with different index contrast.
2. Description of the Related Art
For example, optical waveguides are used within the field of optical communications. The ability of a waveguide to limit the size of the light field perpendicularly to the longitudinal direction of the waveguide depends on its index contrast. Herein, the expression index contrast refers to a measure of the difference in effective index in a lateral direction, diametrically to the main direction of propagation of the light, for the effective index profile of the waveguide when reduced to one dimension. High index contrast leads to a strong limitation of the extension of the light mode in the waveguide and vice versa.
For example, low index contrast waveguides are used in so called ridge lasers, in which these waveguides among other things lead to high reliability and simplified manufacturing. High index contrast waveguides are, for example, used in so called modulators, in which it is possible to affect the refractive index of the waveguides by applying a small voltage, and in which one may reach high efficiency regarding the change as a consequence of the high index contrast.
It is often desirable to mix high index contrast waveguides with low index contrast waveguides in the same technical application, whereby the problem arises of achieving transition parts between such waveguides, which transitions parts would lead to small losses and not to other problems such as for instance unwanted reflections. In particular, this is true for monolithic, integrated optical circuits, in which different waveguides are arranged on the same chip.
Moreover, in certain applications it may be desirable to control the losses in such transition parts with great exactness to certain predetermined values. For example, this may be the case when exact control is desired over the difference in loss between several different transitions, for instance in a modulator.
The American U.S. Pat. No. 5,799,119 discloses a transition part connecting two ridge structure waveguides, in other words waveguides involving a so called ridge structure, with low and high index contrast, respectively (see FIG. 11 in the referred to patent). The transition takes place by the use of a gradually narrowing, contracting or down-tapering of a lower layer in the waveguide, whereby the vertical mode confinement gradually changes across the transition.